Conventionally, there is a known tool post for a machine tool, the tool post transmitting rotational driving force to a tool. The tool post includes a supporting member that movably supports the tool. Between the supporting member and the tool, a driving force transmission part is disposed to transmit rotational driving force to the tool. The driving force transmission part includes an engaging member on the side of the supporting member and an engaging member on the side of the tool. Then engaging members detachably engage with the transmission part in accordance with the movement of the tool. At least one of the engaging members is disposed to be biased to an engaging posture in which both of the engaging members are capable of engaging. Between the tool side and the supporting member side, a releasing member is disposed. The releasing member releases the engaging posture by pressing the engaging members against the biasing force. The tool post is configured to transmit the rotational driving force to the tool by moving the tool to a predetermined position in which the both engaging members engage (Patent Literature 1: JP H07-73804 B, for example).